Low cost container condition monitor

ABSTRACT

Low-cost techniques for sensing ambient temperatures in a container or package using phase change materials are provided. In one aspect, a temperature sensor is provided. The temperature sensor includes: at least one phase change material configured to transition from an amorphous state to a crystalline state upon exposure to temperatures above a predetermined threshold temperature for a given duration. A method of monitoring temperature exposure of a consumer good using the temperature sensor is also provided.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a divisional of U.S. application Ser. No. 15/272,146filed on Sep. 21, 2016, now U.S. Pat. No. 10,436,651, the disclosure ofwhich is incorporated by reference herein.

FIELD OF THE INVENTION

The present invention relates to monitoring conditions in a container,and more particularly, to low-cost techniques for sensing ambienttemperatures in a container or package using phase change materials(PCMs).

BACKGROUND OF THE INVENTION

In most cases, temperature is the most important parameter to monitorwhen handling packed medicine, chemicals, and even food. The rate ofmany chemical reactions is typically exponential with temperature change(see Arrhenius equation). Thus the shelf life of a packaged good and theefficacy of the content can substantially change due to an exposure tohigh temperatures. In some cases, such as with packaged medicine, theexposure to high temperatures can render a drug ineffective, which canlead to dangerous situations.

It is therefore important to monitor and record the package temperaturehistory and, at a minimum, be able to tell if a packaged good wasexposed to a temperature that exceeds the maximum allowed temperature.While a variety of different techniques exist that could be used formonitoring packaging temperatures over time, the cost of the associateddevices is typically too high as compared to the cost of the packagedgoods to justify embedding such a device in each package. Accordingly,there is a need for a monitoring device with a cost so low that it doesnot substantially change the overall price of the packaged goods.

Another issue is that most monitoring devices require a power supply tomaintain their operation. While the inclusion of a battery is possible,it raises the package cost, it limits the monitoring period to that ofthe battery life, and in very cold environments, where packages aresometimes stored, battery operation is limited. These issues make itmore desirable to have a monitoring device that is passive, and does notrequire a power source for operation.

Thus, there is a need for a simple, low cost passive device to monitormaterials in containers to assure that their content is safe and worthyof using.

SUMMARY OF THE INVENTION

The present invention provides low-cost techniques for sensing ambienttemperatures in a container or package using phase change materials. Inone aspect of the invention, a temperature sensor is provided. Thetemperature sensor includes: at least one phase change materialconfigured to transition from an amorphous state to a crystalline stateupon exposure to temperatures above a predetermined thresholdtemperature for a given duration. By way of example only, thetemperature sensor can include multiple phase change materials, eachhaving a different predetermined threshold temperature. Further, the atleast one phase change material can be configured to change reflectivityupon exposure to temperatures above the predetermined thresholdtemperature.

In another aspect of the invention, a method of monitoring temperatureexposure of a consumer good is provided. The method includes: placing atemperature sensor on or near the consumer good, wherein the temperaturesensor includes at least one phase change material configured totransition from an amorphous state to a crystalline state upon exposureto temperatures above a predetermined threshold temperature for a givenduration; and reading the sensor to determine a state of the at leastone phase change material. For instance, the at least one phase changematerial can be patterned into a message that is only visible to a userwhen the at least one phase change material is in a crystalline state,and the temperature sensor is read optically. Alternatively, thetemperature sensor can include a radio-frequency identification (RFID)circuit in which the at least one phase change material is present as aresistor, and the temperature sensor is read electrically based onoutput from the RFID circuit.

A more complete understanding of the present invention, as well asfurther features and advantages of the present invention, will beobtained by reference to the following detailed description anddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating an exemplary methodology for monitoringexposure of a consumer good to high temperature conditions using thepresent phase change material (PCM)-based temperature sensors accordingto an embodiment of the present invention;

FIG. 2 is a comparator circuit used to detect if a package has (at oneor more times) exceeded a maximum temperature according to an embodimentof the present invention;

FIG. 2A is a variation of the comparator circuit having multiple PCMsaccording to an embodiment of the present invention;

FIG. 3 is a frequency shifter circuit used to detect if a package has(at one or more times) exceeded a maximum temperature according to anembodiment of the present invention;

FIG. 4 is a diagram illustrating an exemplary optical readout schemeaccording to an embodiment of the present invention; and

FIG. 5 is a diagram illustrating a smart phone being used as an RFIDreader to obtain data from the temperature sensor according to anembodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Provided herein are low-cost techniques for sensing ambient temperaturesin a container or package. The present sensors employ a class ofmaterials known as phase change materials (or PCMs) to sense the ambienttemperature in a package. Advantageously, the sensor is passive and therecording of the temperature is accomplished by a change in the phase ofthe PCM. The phase of the PCM is read optically by observing a change inreflectivity and/or it can be read electrically by powering a simpleradio-frequency identification (RFID)-type of circuit electricallyconnected to the PCM sensing element.

The present techniques leverage the notion that the optical propertiesand/or electric resistance of PCMs can be changed when the PCM isexposed to a certain range of temperatures over a certain range of time.The use of such materials enables battery-less monitoring at a very lowcost.

PCMs typically have two phases. In a first phase (an amorphous phase)the material has high resistivity (as compared to the crystallinephase—see below). When the material is heated up above some thresholdtemperature (which varies depending on the particular PCM beingemployed), the material transitions into a second phase (a crystallinephase). The crystalline phase is characterized by a lower resistance(i.e., as compared to the amorphous phase). Cooling down the materialdoes not revert the phase. Thus, PCMs are well suited to documenttemperature excursions above their respective transition thresholds.

PCMs are available at a large range of threshold temperatures for theirphase transition. Thus, a vast selection of candidate PCMs are availableto pick from for a particular application(s) and a given temperaturethreshold(s). By incorporating the PCM in a sensor that is embedded inthe package, it is easy to tell if the package temperature has at somepoint exceeded a given transition threshold of the select PCM.Furthermore, embodiments are anticipated herein where multiple PCMs withdifferent transition thresholds are employed in the same package. Byembedding PCM materials with different threshold temperatures the sensorcan report a range of temperatures that the package has been exposed to(e.g., by indicating what range of transition threshold temperatures hasbeen exceeded).

To use a simple example to illustrate this concept, say the sensorembedded in the package contains PCMs A, B, C, and D, which havetransition temperatures T_(A), T_(B), T_(C), and T_(D), respectively,wherein T_(A)<T_(B)<T_(C)<T_(D). If upon reading the sensor it is foundthat the PCMs A and B are in the crystalline phase, and C and D are inthe amorphous phase, then it may be deduced that the package has seentemperatures exceeding T_(A) and T_(B), but perhaps not as high as T_(C)or T_(D). However, if only one PCM A was used in the sensor, and it iscrystalline, then at most one would know that the package temperaturehas exceeded T_(A), but not by how much.

PCMs can differ not only in the threshold temperature in which phasechange occurs, but also in the speed at which such changes occur. Thus,advantageously, the PCM material(s) used in the present sensor can beselected such that phase change occurs only after the container orpackage has been exposed to a certain temperature for a minimal duration(which can be predetermined based on the phase change properties of thePCM(s) selected for the sensor). Being able to set a duration boundaryon temperature excursions might be useful, for example, when monitoringcontainers of food. Bacteria proliferate when environmental conditionsare suitable, such as an ambient temperature above a certain threshold.Bacterial proliferation can reduce the shelf life of products and posehazards to consumers. However, proliferation of microorganisms may notoccur if the temperature threshold is crossed only briefly. Therefore,according to an exemplary embodiment, when monitoring the suitability ofgoods such as food, the PCM(s) is/are engineered to change to thecrystalline phase only after a predetermined duration of hightemperature exposure.

Further, the transition to a crystalline phase will complete only aftera cumulative time exposure to the predetermined elevated temperature. Itis notable that if multiple short exposures to elevated temperatureoccur, the PCM material integrates the exposures. As a result, N shortexposures to high temperature of duration t_(expo), would result in asimilar change to the PCM as a single exposure of duration N*t_(expo).Another way to look at this is say, for instance, that the PCM isexposed to momentary temperatures above the threshold (i.e., for aduration not long enough to transition the PCM to the crystallinephase). After the exposure, the PCM does not go back to its pre-exposurestate. Instead, the next short exposure will put the PCM closer to atransition to the crystalline phase, and so on, until the transitionoccurs. As a result the PCM will provide a reliable indicator for foodspoilage if the bacteria growth has short incubation time compared withthe exposure time at elevated temperature.

Therefore, the parameters measureable via the present PCM-based sensorinclude, but are not limited to, temperature excursions over apredetermined temperature(s), a duration of the temperature excursionsand/or an accumulation of short duration exposures. As provided above,these parameters can be measured based on the configuration of the PCMor, preferably, multiple PCMs, included in the sensor. For instance, asprovided above, multiple PCMs can be used to establish a range oftemperature variations.

FIG. 1 is a diagram illustrating an exemplary methodology 100 formonitoring exposure of a consumer good to high temperature conditionsusing the present PCM-based sensors. Accordingly, consumer goods mostbenefited by this process are those that are sensitive tohigh-temperatures (i.e., temperatures above a certain thresholdtemperature). Of course, the goods that fit into this category are vast.Therefore, as a representative group, some examples oftemperature-sensitive consumer goods include, but are not limited to,foods, chemicals, medications, reagents, electronic hardware, clothing,art pieces, etc.

In step 102, the present PCM-based sensor is placed on or in closeproximity to the consumer good. For instance, in one exemplaryembodiment, the PCM-based sensor is placed in or on the same package orcontainer as the consumer good. By way of example only, the packagingcan include a box, a bag, a bottle, a flask, an ampule, a syringe, atest tube, a tray, a multi-well plate, etc. For instance, when theconsumer good is a food item in a container, then the PCM-based sensorcan be placed like a label on the outside of the container, or withinthe container.

Alternatively, according to another exemplary embodiment, the PCM-basedsensor might be affixed directly to the consumer good. For example, whenthe consumer good is an article of clothing that would be damaged byhigh temperatures (such as during washing, ironing, etc.), then thePCM-based sensor can be fashioned as a label that is affixed directly tothe clothing.

As provided above, the PCM-based sensor can include one, or preferably,multiple PCMs with different temperature transition thresholds. Withmultiple PCMs, a range of temperatures and/or duration or exposure canbe detected. An example implementing multiple PCMs with differentthreshold temperatures was provided above. With regard to the durationof exposure, the same general principle applies. For instance, multiplePCMs can be employed that require different durations of exposure (abovea threshold temperature) to transition from the amorphous to thecrystalline phase. To use a simple example to illustrate this concept,say the sensor contains PCMs a, b, c, and d, which require differentdurations of high-temperature exposure (above the thresholdtemperature), i.e., duration a, duration b, duration c, and duration d,respectively, for transition, and wherein duration a<duration b<durationc<duration d. If upon reading the sensor it is found that the PCMs a andb are in the crystalline phase, and c and d remain in the amorphousphase, then it may be deduced that the good has seen high-temperaturesfor a duration exceeding duration a and duration b, but perhaps not aslong as duration c or duration d.

In step 104, readout of the sensor is performed to determine theconditions to which the good has been subjected. As provided above,using the present PCM-based sensors, these conditions can includetemperature and/or range of temperatures the good has been subjected to,and preferably the duration of the exposure (optionally also as arange).

Readout in step 104 can be performed at any stage of handling, such asduring storage of the good (pre- or post-sale) by the manufacturer,during shipping and/or transport, post-purchase by the consumer, etc.Namely, due to the simple and convenient readout methods describedherein, the state of the sensor can be easily ascertained by any partyinvolved in handling of the good, without any specialized equipment. Forinstance, in one exemplary embodiment, an optical readout method isanticipated where the PCM-based sensor is incorporated into a label thatchanges visually (e.g., perceptive to someone simply looking at thelabel) if/when the PCM transition occurs. For instance, a user-friendlydesign is described below, where the PCM is configured as a message tothe user that is set initially against a same color background, and thusnot visible to the user. However, upon transition to the crystallinephase the reflective properties of the PCM change relative to thebackground, and the message becomes visible to the user. To use a simpleexample, the PCM message “NOT” can be placed before the phrase “SAFE TOCONSUME” on the label. However, when in the amorphous form the PCMmessage is set against the same color background, and thus not visibleto the consumer. Thus all the consumer sees is the phrase “SAFE TOCONSUME.” Upon transition (due to high-temperature exposure), the PCMvisibly changes making it distinguishable from the background. In thatcase, the consumer now sees the message “NOT SAFE TO CONSUME.” Thisexemplary embodiment is described in further detail below.

Another readout method anticipated herein (which optionally can be usedin conjunction with the above-described optical readout) is anelectrical readout method. As will be described in detail below, asimple passive RFID circuit which integrates the PCM material(s) as aresistor can be employed, which is readout using a simple comparatorand/or frequency shifter circuit. Advantageously, use of RFID technologyeliminates the need for battery or other sensor power source forreadout. To obtain data from the RFID circuit, all that is needed is anRFID reader placed near the sensor. A standard RFID reader can be usedfor readout or, as described below, a properly equipped smart phone(e.g., a smart phone with near-field communication (NFC) capabilities)is also envisioned as a possible RFID reader. Advantageously, being ableto interface with RFID technology via a smart phone further enables useof the present techniques in situations where consumers readily havesmart phone technology at their disposal to extract data from thesensors, such as at home or at work, while shopping, etc.

In step 106, a determination is made as to whether the readout ispositive—i.e., whether the sensor indicates high-temperature exposure.If the sensor reading (YES) is positive, meaning that the good has beenexposed to high temperatures, then the process ends. The consumer orother handler of the good can then determine what to do with the exposedproduct, e.g., discard, return for a refund, exchange for new product,etc. On the other hand, if the sensor reading (NO) is negative, meaningthat the good has not been exposed to high temperatures, then theprocess can be iterated at one or more later times to keep track of theconditions. For instance, a manufacturer might make periodic checks oftheir products, as might the shipper, retailer, and/or end-userconsumer, etc. All of these parties are considered potential users ofthe present techniques.

As provided above, readout of the present PCM-based sensor can beperformed electrically via a passive RFID circuit in which the PCM(s)is/are incorporated as a resistor. One example of such a circuit is asimple comparator circuit 200 as shown in FIG. 2.

As shown in FIG. 2, circuit 200 includes an operational amplifier 202that compares the input from two pairs of ‘resistors,’ one pair includestwo resistors and the other includes a resistor in series with thePCM(s) as the equivalent of a resistor. Namely, as described above, whenin the amorphous form the PCM acts as a resistor. Thus, initially, theinput to operational amplifier 202 is the same from both pairs ofresistors.

However, when exposed to high-temperatures the PCM changes to itscrystalline phase which has low resistivity. As a result of thisexposure and phase change, the resistance across the 2^(nd) pair ofresistors changes, and there is a voltage drop on the (+) and (−) inputsto the operational amplifier 202. As a result, the light emitting diode(LED) or other similar (visual) indicator which is connected to theoutput of the operational amplifier 202 will light up. Illumination ofthe LED indicator will occur when an RFID reader is near the sensorcircuit. Specifically, according to an exemplary embodiment, the ‘powersource’ in circuit 200 comes from the energy harvested from a radiofrequency (RF) electromagnetic field emitted by an RFID reader. Seebelow. The RFID reader might be a hand-held conventional RFID reader(which can be brought to the sensor) or a stationary unit (to which thecontainer or package containing the sensor can be brought) or, asdescribed in detail below, the RFID reader can be a smart phone therebyenabling consumers to leverage the technology generally at theirdisposal to read the sensor. Furthermore, even with smart phones thatare not equipped with RFID capabilities, energy from RF electromagneticfield emitted by the device (e.g., WiFi and Bluetooth®) can be harvestedto power the circuit.

As described above, the present sensor design can include multiple PCMswith different threshold temperatures, exposure duration thresholds,etc. To incorporate multiple PCMs into the comparator circuit design,one can simply duplicate the comparator circuit 200 with each versioncontaining a different PCM. That way, during readout, one can read theresults of the various PCMs based on which of the LEDs is lighted. TheLEDs can be labeled, color-coded, or in another way distinguished as towhich of the PCMs they are linked. See, for example, FIG. 2A.

In the example depicted in FIG. 2A, the comparator circuit 200 isreplicated in triplicate (i.e., 200 a, 200 b, and 200 c) with fourdifferent PCMs, i.e., PCM1, PCM2, and PCM3. The corresponding LEDs arecolor-coded as Red, Green, and Blue. Thus, if the transitiontemperatures of the PCMs (T_(PCM)) are T_(PCM1)<T_(PCM2)<T_(PCM3), thenduring readout a Red and Green LED indicates that the good has beenexposed to temperatures exceeding T_(PCM1) and T_(PCM2) but not as highas T_(PCM3).

Another exemplary circuit that permits electrical readout of the presentPCM sensor is a frequency shifter circuit 300 as shown in FIG. 3. In thesame general manner as the comparator circuit 200, a PCM is theequivalent of a resistor in the frequency shifter circuit 300 and itstransition from a high-resistivity amorphous state to low-resistivitycrystalline state upon exposure to high temperatures affects the output(in this case the frequency of the output from the circuit.Specifically, as shown in FIG. 3, the PCM is included in a phase shiftfeedback loop of capacitors (C) and resistors (R) (having a phase shiftof 60°, 120°, and 180°), wherein the oscillation frequency depends onthe product of R*C. Thus, the state of the PCM (the equivalent of one ofthe resistors) shifts the frequency of the signal through, in this case,operational amplifier 302. The output of the sensor can be ascertainedsimply by reading the oscillation frequency of the circuit, which willshift based on the phases of the PCM output frequency. For instance,when an RFID reader is placed near the sensor (or vice versa), thereturned RF signal is modulated by the output frequency from the circuit300.

As with the layout provided in FIG. 2A, duplicates of the circuit 300can be employed, each having a different PCM. In that manner, thepresent sensor design can include multiple PCMs with different thresholdtemperatures, exposure duration thresholds, etc.

Suitable PCMs include, but are not limited to, chalcogenide alloys. Forinstance, according to an exemplary embodiment, the PCM is an alloycontaining the chalcogenide element tellurium (Te) in combination withother elements such as antimony (Sb) and/or germanium (Ge), formingalloys such as SbTe, GeTe, and GeSbTe. The properties of these PCMalloys can vary depending on their composition. For instance, taking anSbTe PCM as an example, the onset temperature for the phase change canbe tuned by changing the Sb:Te ratio. See, for example, Morales-Sanchezet al., “Crystallization of SbTe Phase Change Optical Films,” 4^(th)International Conference on Electrical and Electronics Engineering, 2007(September 2007) (hereinafter “Morales-Sanchez”), the contents of whichare incorporated by reference as if fully set forth herein. Forinstance, as described in Morales-Sanchez, tuning the phase change tooccur at a variety of different temperatures ranging from about 60° C.to about 100° C. can be achieved simply by increasing the Sb:Te ratio.

With regard to duration, for long durations the transition time can betuned by the temperature margin between the storage temperature and thephase transition temperature. For short durations, the transition timecan be controlled by controlling the mechanism that governs thecrystallization (growth-dominated material versus nucleation-dominatedmaterial).

Another useful property that can be tuned based on the composition ofthe PCM is the reflectivity of the material. As highlighted above, thePCM material readout can also be done optically, and reflectivity canchange how the PCM appears to the user. For instance, Morales-Sanchezdescribes the measured reflectivity of films of antimony tellurideSb(x)Te(1-x) as a function of the film temperature. The films observedin Morales-Sanchez included Sb₈₃Te₁₇, Sb₈₁Te₁₉, Sb₇₅Te₂₅, Sb₇₁Te₂₉, andSb₆₇Te₃₃. In the amorphous phase, the measured reflectivity of all thefilms is low. Taking for example the Sb₈₃Te₁₇ film, as the filmtemperature reaches about 60° C. the measured reflectivity abruptlyincreases indicating that the phase changed to crystalline. It isnotable that if the film is subsequently cooled to a temperature below60° C. the phase will remain crystalline. As such the sensor willmaintain the temperature history, and the event of exceeding a maximumallowed temperature will be stored in the film for later readout byoptical or electrical means.

Reflectivity affects the color objects appear to a user. Thus, ifreflectivity of the PCM changes above the threshold temperature, thenthe color of the PCM also changes. This notion can be leveraged for anoptical readout scheme. In its simplest form, a label can be createdfrom the PCM and, depending on the color of the label, the user knowswhether high-temperature exposure has occurred indicating that thecontainer content should not be used.

A preferred, user-friendly approach is also contemplated herein wherethe PCM is present in the shape of a message (e.g., letters or words)that conveys meaning to a user upon reading the message. For instance,the PCM can be patterned into the shape of the message (e.g., the PCM ispatterned into various letters and/or words). Alternatively, a patternedmask can be placed over the PCM, where the mask is patterned withvarious letters and/or words, and where the PCM is visible through thepatterned letters and/or words). As described above, the message can beset against a background color that is the same as the PCM message inthe amorphous form. However, upon exposure to temperatures above thethreshold, the reflectivity of the PCM message will change relative tothe background. Thus, the PCM message will appear as a different colorthan the background, and thus will now be visible to the user who looksat the label. See, for example, FIG. 4. In the example depicted in FIG.4, the label is preprinted with the message “SAFE TO CONSUME” and thePCM message is simply the word “NOT” set against a same color backgroundwhen the PCM message is in the amorphous state. Thus, the user will onlysee the message “SAFE TO CONSUME.” However, after high-temperatureexposure and transition to the crystalline state, the PCM message “NOT”will become visible before the message “SAFE TO CONSUME.” The user willthen know that the container content should not be used. When apatterned mask is used over the (unpatterned) PCM, the mask preferablyhas the same color as the PCM in the amorphous form and the background.Thus, prior to exposure, the mask is indistinguishable from the PCM andthe background. However, upon exposure, the reflectivity of the PCM willchange relative to the mask and background thus making the PCM visiblefrom behind the mask (through the message patterned in the mask).

As provided above, readout of the sensor can be performed optically(e.g., by simply viewing a PCM message set against a selectivebackground, looking at an indicator LED, etc.) and/or electrically viaan RFID reader that is brought near the sensor or vice versa. This RFIDreader can be a conventional reader or, alternatively, a smart phone.

When readout is performed using a smart phone, the readout of the sensoris displayed on the smart phone screen. This is different than the otherembodiments provided above where the readout was attached to thecontainer or package such as the light of an LED goes on or is off, orthe optical readout, such as the text “NOT” preceding the text “SAFE TOCONSUME,” etc.

In this example, the energy for the operation of the circuit (see, forexample, circuits 200 and 300—described above) that “reads” the PCMresistor is harvested from the radio frequency RF (electromagneticfield) emitted by the smart phone. For example, in one exemplaryembodiment, that harvested RF energy is used to operate a comparatorcircuit (such as circuit 200—described above) that can light up a LED.However, rather than powering a LED, one can power a transmitter circuitthat sends the “good/bad” information (which is based on the sensorbeing in one of two states “PCM resistor is crystalline/PCM resistor isamorphous”) to a receiver in the smart phone. The receiver in the smartphone receives the information and, through an application, the phonewould display, for example, “GOOD TO USE” or “NOT GOOD TO USE” dependingon the received information.

Some smart phones come equipped with such hardware (receiver andtransmitter) as they use NFC which is a specialized type of RFID.However, even if a given smart phone does not have full RFIDcapabilities it still can be used to provide RF energy to power up thesensor circuit.

As for the frequency shifter circuits (such as circuit 300—describedabove), the same principles apply. The frequency can be used to modulatean RF signal and the receiver on a smart phone can demodulate thatfrequency from the RF signal. This is just another way to transmitinformation such as in frequency modulation (FM) or amplitude modulation(AM).

See, for example, FIG. 5 which schematically illustrates a smart phonebeing used as an RFID reader to obtain data from the temperature sensor.Specifically, when the smart phone is bought near the sensor, energy foroperation of the circuit that “reads” the PCM resistor is harvested fromthe radio frequency RF (electromagnetic field) emitted by the smartphone. The receiver in the smart phone receives the information and,through an application, the phone displays a message to the smart phoneuser, in this case: “GOOD TO USE.”

Although illustrative embodiments of the present invention have beendescribed herein, it is to be understood that the invention is notlimited to those precise embodiments, and that various other changes andmodifications may be made by one skilled in the art without departingfrom the scope of the invention.

What is claimed is:
 1. A temperature sensor, comprising: at least onephase change material configured to transition from an amorphous stateto a crystalline state upon exposure to temperatures above apredetermined threshold temperature for a given duration; and aradio-frequency identification (RFID) circuit in which the at least onephase change material is present, wherein the RFID circuit comprises acomparator circuit with an operational amplifier having a first inputconnected between a pair of resistors that are connected in series and asecond input connected between a resistor and the at least one phasechange material connected in series.
 2. The temperature sensor of claim1, wherein the at least one phase change material comprises tellurium incombination with an element selected from the group consisting of:antimony and germanium.
 3. The temperature sensor of claim 2, whereinthe at least one phase change material comprises tellurium and antimony.4. The temperature sensor of claim 1, comprising multiple phase changematerials each having a different predetermined threshold temperature.5. The temperature sensor of claim 1, further comprising a lightemitting diode (LED) indicator connected directly to an output of theoperational amplifier.
 6. A method of monitoring temperature exposure ofa consumer good, the method comprising: placing a temperature sensor onthe consumer good, or in or on a same container as the consumer good,wherein the temperature sensor comprises at least one phase changematerial configured to transition from an amorphous state to acrystalline state upon exposure to temperatures above a predeterminedthreshold temperature for a given duration, and an RFID circuit in whichthe at least one phase change material is present, wherein the RFIDcircuit comprises a comparator circuit with an operational amplifierhaving a first input connected between a pair of resistors that areconnected in series and a second input connected between a resistor andthe at least one phase change material connected in series; and readingthe temperature sensor to determine a state of the at least one phasechange material by reading the temperature sensor electrically based onoutput from the RFID circuit.
 7. The method of claim 6, wherein the atleast one phase change material comprises tellurium in combination withan element selected from the group consisting of: antimony andgermanium.
 8. The method of claim 7, wherein the at least one phasechange material comprises tellurium and antimony.
 9. The method of claim6, wherein the temperature sensor comprises multiple phase changematerials each having a different predetermined threshold temperature.10. The method of claim 6, wherein the temperature sensor is placed inthe container.
 11. The method of claim 10, wherein the container isselected from the group consisting of: a box, a bag, a bottle, a flask,an ampule, a syringe, a test tube, a tray, and a multi-well plate. 12.The method of claim 6, wherein the consumer good is selected from thegroup consisting of: foods, chemicals, medications, reagents, electronichardware, clothing and art pieces.
 13. The method of claim 6, whereinthe temperature sensor is affixed as a label to the consumer good. 14.The method of claim 6, further comprising: reading the temperaturesensor using an RFID reader.
 15. The method of claim 14, wherein theRFID reader comprises a smart phone.